The invention generally relates to a downhole magnetic-field based feature detector for detecting features of a downhole pipe.
Certain downhole oilfield applications, such as perforating applications, require the ability to be able to position a tool at a particular and known spot in the well. For example, a wireline (armored electric cable) service uses a tool assembly (e.g., instrument) that is lowered downhole via a wireline. A depth counter may be used at surface to track the length of the dispensed cable to approximate the depth of the tool assembly. However, because the depth counter does not precisely indicate the depth (primarily because of stretch in the cable), other techniques may be used.
For example, a more precise technique may use a depth control or depth correlation log (e.g., casing collar locator log), a log that is run while ascending and descending in the well indicates the depths of various casing collar joints of the well. In this manner, the well includes casing collar joints, joints at which casing segments are coupled together to form the well casing. Each casing collar joint includes a casing collar to couple two adjacent casing segments of the well casing together. An air gap may exist between the ends of adjacent casing segments.
To obtain the depth control log, a wireline tool assembly may be run downhole and include a detection device, called a casing collar locator, to detect the casing collar joints. When the casing collar locator indicates detection of a casing collar joint, the coarse depth that is provided by the depth counter may be used to locate the corresponding casing collar joint on the depth control log. Because the depth control log precisely shows the depth of the detected casing collar joint, the precise depth of the tool assembly may be determined. From this determination, an error compensation factor may be derived. Then, when a perforating gun is positioned downhole, the error compensation factor is used to compensate the reading of the depth counter to precisely position the gun.
A conventional casing collar locator is a passive device that uses the principle of magnetic inductance to detect casing collar joints. In this manner, the casing collar locator typically includes an electrical coil, or winding, through which a magnetic flux field that is created by one or more permanent magnets passes. When a change occurs in the effective magnetic permeability in the surrounding, such as in the presence of a casing collar joint, a voltage is induced on the coil winding due to the corresponding change in the magnetic flux field (disturbance). Therefore, as the casing collar locator passes the casing collar joint, the change in permeability (caused by such things as the presence of the air gap between adjacent well casing segments and the casing collar) causes a change in the magnetic flux field to induce a signal across the winding. This generated signal may be communicated uphole and observed at the surface of the well. Thus, with this technique of detecting casing collar joints, the casing collar locator must be in motion to produce the signal.
The quality of the signal may be highly dependent on the degree to which the magnetic permeability changes, or is disturbed. In this manner, the higher the rate of change in the permeability that is experienced by the magnetic flux field, the higher the induced signal (to a finite degree). The degree to which the field is disturbed depends on such factors as the distance, or gap (also called the xe2x80x9cstand-offxe2x80x9d), between the casing collar locator and the casing; the magnetic properties (i.e., the permeability) of the surrounding well casing; and the degree of change in geometry or bulk-mass of the casing, i.e., the change must be drastic enough and abrupt enough to cause a rapid enough disturbance in the flux field.
If the field is not sufficiently disturbed, the resulting signal may be too small to be detected at the surface. The signal-to-noise ratio of the signal produced downhole typically places a limit on the degree to which the signal can be boosted, or amplified. Therefore, for these reasons, it may be very difficult to detect joints of casing that is made from a material having a low magnetic permeability, such as Hastalloy, for example. Likewise, collar joints that have no casing collars are difficult to detect, particularly if the joints are xe2x80x9cflushxe2x80x9d (i.e., each joint has no or almost no air gap).
Another difficulty associated with a conventional casing collar locator is its mass and size. In this manner, the conventional casing collar locator may include two or more permanent magnets, one or more coils, and one or more coil cores, or bobbins. The combination of all of these components imparts a large mass to the casing collar locator. This large mass, in turn, may cause a significant force to be exerted on the casing collar locator during perforating operations due to the high acceleration and shock that is placed on this large mass. This force may damage the casing collar locator if extensive measures are not undertaken to properly pack the casing collar locator in the string.
Besides having a large mass, the casing collar locator typically is quite bulky, as the locator may extend from six to eighteen inches and beyond, not including the pressure housing and connections. The tool string that houses the casing collar locator is therefore long and cumbersome. Tool length is very important particularly when the tool string is conveyed on a wireline and when working with high well pressure. Having a long tool string can present major operational and safety problems with pressure control equipment, such as the lubricator and riser pipe. Therefore, it is typically important to conserve every inch of a tool string, particularly in perforating applications.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.
In an embodiment of the invention, an apparatus that is usable with a subterranean well includes a magnetometer and a circuit. The magnetometer indicates a strength of a magnetic field that at least partially extends through a portion of a downhole pipe. The circuit is coupled to the magnetometer to indicate a feature present in the pipe based on the indication from the magnetometer.
In another embodiment of the invention, an apparatus that is usable with a subterranean well includes a magnet and a winding. The magnet establishes a flux field near the apparatus, and the flux field at least partially extends through a portion of a downhole pipe. The winding generates a signal produced by a change in a strength of the flux field to indicate a feature of the pipe. The longitudinal dimension of the apparatus does not exceed approximately two inches.
In yet another embodiment of the invention, an apparatus that is usable in a subterranean well includes a first winding, a second winding, a powered interface and an unpowered interface. The first winding generates a first signal in response to a change in a magnetic field that at least partially extends through the first winding and at least partially extends through a portion of a downhole pipe to indicate a feature of the pipe. The second winding generates a second signal in response to a change in the magnetic field to indicate detection of the feature of the pipe. The magnetic field at least partially extends through the second winding. The first interface is coupled to the first winding to communicate the first signal to the surface of the well when the apparatus is in a powered mode, and the second interface is coupled to the second winding to communicate the second signal to the surface of the well when the apparatus is in an unpowered mode.
Additional advantages and other features of the invention will become from the following description, drawing and claims.